1. Field of the Invention
The present invention relates to a metal oxide semiconductor (MOS) device and fabricating method thereof, and particularly to a power MOS field effect transistor (Power MOSFET) and fabricating method thereof.
2. Description of the Related Art
A conventional Power MOSFET is usually designed as a vertical transistor, wherein a drain is formed on the back face of a wafer, and a source and a gate of transistors are formed on the front face of the wafer. Hence, the drains of the transistors are connected in parallel so as to endure considerably large driving-current.
Based on a conventional method of fabricating Power MOSFET, a substrate doped by n-type dopant is first provided to be used as a drain. An epitaxial layer is formed on the substrate, and then the epitaxial layer is patterned to form trenches and islands. Next, a gate oxide layer and a gate are formed in the trench, and a doping layer is formed on the surface of the islands. Ions in the doping layer are diffused to the islands via a thermal drive-in mechanism so that the p-type dopant is doped in the islands and consequently a source is formed at the top of the islands.
FIG. 1 shows doping concentration in a device of a conventional Power MOSFET. The vertical coordinate axis indicates concentration and the horizontal coordinate axis indicates depth. A drain (n-type) is located on the bottom, and a source (n-type) is located at the top of the device. The doping concentration between the source, the islands (p-type) and the drain decreases gradually in the direction from the source to the drain, and the highest concentration is located near the source. This is because that the p-type ions are doped in the islands conventionally via a thermal drive-in mechanism, so that the ion concentration is higher near the source (i.e., the top of the islands).
Therefore, there exists a problem in such a concentration distribution in the islands of the devices. During the operation of the devices, a high voltage is applied on the drain, depletion will develop gradually in the direction from the drain towards the source, and consequently depletion region will be generated between the drain and the source. The width of the depletion region increases with the drop in doping concentration in the depletion region. If the depletion region becomes too wide, a punch through will occur between the source and the drain. As shown in FIG. 1, however, the ion concentration in islands of conventional devices decreases in the direction from the source to the drain, and the highest concentration is near the source. In the region near the drain, the concentration is low and thus a large depletion region will be formed. For increasing punch through voltage of the devices, it requires to increase the distance (channel length) between the source and the drain, which will increase the depth of the trench and consequently increase the channel resistance.
Moreover, the threshold voltage of a device of conventional Power MOSFET depends on the highest concentration of doping concentration in the islands. On one hand, the doping concentration near the drain can be increased through the increase of the doping concentration in the islands, which may prevent the depletion region from becoming too wide, but will cause the increase of the threshold voltage of the device, reduce the driving current of the device, and consequently decrease the mobility of the carriers and increase the channel resistance. On the other hand, the doping concentration near the source can be lowered through decreasing the doping concentration in the islands, but which will cause the drop of the threshold voltage of the device, further decrease the doping concentration near the drain, and consequently enlarge the depletion region. Therefore, the doping concentration in the islands of a conventional device affects the threshold voltage of the device, and the magnitude of the threshold voltage will affect the doping concentration near the source and the drain and further affect the channel length or the channel resistance.